I think one at a time is usually easiest, but if you start to deal with wanderlust (like I often do), sometimes it can be helpful to add another one. Even then, I figure it's best to wait until you've been studying the first language for at least some months or a year.

Back then I was greedy and wanted to learn as much languages as possible. However in my workplace there is someone who also like to learn many languages, and despite of that, he said that he wanted to only focus on two: French and German. Then I also decided to focus on two: Arabic and Japanese, because I have started learning them ever since I was an elementary student.